Asymmetric cell division is an essential process in the development of diverse animals, including humans. Our stem cells must undergo repeated asymmetric cell disivions to produce ceils of critical importance to our health. The long-term objective of the proposed project is an understanding of how divide asymmetrically. We are using the nematode CaenorhabdiUs elegans as a model to study asymmetric cell division. Before the first cell division in C. elegans, P granules (ribonucleoprotein complexes that are essential for germline development) and several other proteins and mRNAs become asymmetrically localized to one end of the zygote or the other. Simultaneously, actin-based polarized flows of cortical and central cytoplasm occur. Later, during mitosis, the mitotic spindle moves to an asymmetric position. The result is unequal-sized daughter cells containing distinct molecular components. The potential for combining genetics with live imaging makes this system an excellent model for addressing questions about how cells divide asymmetrically. Our specific aims are to (1) determine how known genes function in the localization of P granules, (2) investigate the mechanism of asymmetric mitotic spindle positioning in the C. elegans zygote, and (3) clone and characterize a new gene required for both P granule and mitotic spindle positioning. We expect that we will learn important new information about how cells divide asymmetrically that will suggest mechanisms and guide studies in mammals, including humans.